1. Field of the Invention
The present invention relates generally to signal processing, and more particularly, to a signal processing apparatus for performing active noise control.
2. Description of the Prior Art
The concept of active noise control method is to generate an ‘anti-noise’, which has an amplitude that is substantially identical to a noise source in the environment, but is substantially opposite in phase to the noise source (in practice, the anti-noise may only be similar to the noise source in the low frequency part). By superposition of the sound wave, the noise source and the anti-noise destructively interfere with each other, thereby eliminating the noise. This technology is generally used in a variety of loudspeaker devices, such as headphones. When a user is listening to audio materials via a loudspeaker, the loudspeaker device simultaneously produces the anti-noise by mixing an audio signal corresponding to audio materials with a noise cancellation signal corresponding to the anti-noise. As a result, the user will not be aware of the noise, and the listening experience will be improved. Conventionally, active noise control technology can be implemented by the circuit shown in FIG. 1.
A conventional noise cancellation apparatus illustrated in FIG. 1 includes an acoustic-to-electric transducer 11 (e.g. a microphone), an analog-to-digital converter 12, a filtering circuit 13 and a digital-to-analog converter 14. The acoustic-to-electric transducer 11 is employed for recording noises in the environment, and uses piezoelectricity generation to generate an electrical analog noise signal. The analog-to-digital converter 12 converts the analog noise signal into a digitalized noise signal. The digitalized noise signal will be passed to the filtering circuit 13, which filters the digitalized noise signal based on a transfer function depending on how much of the noise is actually received by the user, to generate a noise cancellation signal which is used to destructively interfere with the noise. An output of the filtering circuit 13 may be further converted to an analog signal by the digital-to-analog converter 14. The analog signal will be processed by an electric-to-acoustic transducer 15 to generate an analog noise cancellation signal. The analog noise cancellation signal will be mixed with an audio signal intended for playback by a mixer 16. After mixing, the user will be unaware of noises in the environment while listening to the audio.
This circuit architecture has certain problems, however. For example, under the consideration of signal gain, the analog signal may be processed by more than one amplifying stage, which may include inverse amplifying stages, before being transmitted to the mixer 16 or the electric-to-acoustic transducer 15. This may cause the analog noise cancellation signal to be inverted twice, which will result in the signal constructively interfering with the noise. Since the purpose of the analog noise cancellation signal is to destructively interfere with the noise, the conventional noise cancellation apparatus is unable to resolve this problem.